This invention relates to a picture encoding method, a picture encoding apparatus and a picture recording medium. More particularly, it relates to a picture encoding method and apparatus and a picture recording medium employed in a system for encoding video signals of a motion picture for storage on a picture recording medium, such as an optical disc, a magnetic disc or a magnetic tape, or in a system employed for transmitting video signals of a moving picture over a transmission channel.
FIG. 1 shows an encoder in which data is compressed as intra-frame coding exploiting line correlation is switched to inter-frame coding exploiting inter-frame correlation or vice versa for improving the transmission efficiency. With the present encoder, prediction of motion compensation is executed on, for example, the macro-block basis for further improving the efficiency.
FIG. 2 shows a decoder as a counterpart device of the encoder of FIG. 1. By employing this decoder, data encoded by high efficiency encoding can be reproduced appropriately.
With the above encoder, the encoding bit rate of a bitstream generated in an encoder 75 is constant for meeting with the transfer rate of a transmission medium 90. The amount of generated data, that is the quantization step size of a quantizer 106, is controlled under these restrictions. Stated differently, if plural pictures with complex patterns occur contiguously, the quantization step size is increased for suppressing the amount of generated data, whereas, if plural picture of simple patterns occur contiguously, the quantization step size is decreased for increasing the amount of generated data for prohibiting overflow or undeflow of the buffer memory 109 from occurring for maintaining a fixed rate.
If, with the above encoder, complex pictures occur contiguously, the quantization step size is increased, thus deteriorating the picture quality, whereas, if simple pictures occur contiguously, the quantized step size is decreased, thus allowing to produce pictures of uniform picture quality on the whole.
On the other hand, for recording a bitstream on a picture recording medium having a limited data capacity, a high fixed rate which will not impair the picture quality has to be applied comprehensively for avoiding deterioration of the complex picture quality, thus decreasing the recording time.
There are also occasions wherein, even if the degree of picture complexity is the same, the encoded picture gives a poor subjective impression as to the picture quality depending upon the picture patterns owing to characteristics of the visual sense of the human being. For example, the encoding noise, such as block noise or mosquito noise, become apparent in dark portions, red portions or flat portions of a picture.